The invention relates generally to flexure-controllable rolls for treating webs of material and, more particularly, to an improved apparatus and method for regulating temperature in flexure-controllable roll having hydrostatic support elements.
In DE-OS 2230139, a flexure-controllable roll is disclosed having a rotatable hollow roll forming the working roll circumference and a stationary crosshead extending lengthwise through the roll to form a surrounding clearance space with the inner circumference of the hollow roll. Piston-type hydraulic support elements are guided for movement in respective cylinder bores of the crosshead. From a cylinder chamber formed under each support element, throttling ports lead into bearing pockets formed in the contact surface of each support element that abuts the inner circumference of the hollow roll. The pressurized fluid supplied to the cylinder chamber flows via the throttling ports into the bearing pockets and over the edges of the bearing pockets into the clearance space between the inner circumference of the hollow roll and the crosshead. Thus, a continuously replenished liquid film develops between the edges of the bearing pockets and the inner circumference of the hollow roll. In this manner, both the center and edge regions of the bearing pockets of the support element are braced against the inner circumference of the hollow roll by the pressurized fluid.
As the pressure in the bearing pockets may be quite high, the pressurized fluid supplied to the bearing pockets flows from the bearing pockets to the outside under high pressure via the very narrow gap formed between the edges of the bearing pockets and the inner circumference of the hollow roll, as noted above. The pressurized fluid becomes heated as it flows from the bearing pockets, while the temperature of the hollow roll is determined mainly by the pressurized fluid supplied to the cylinder chambers of the piston/cylinder units of the support elements, additional heat generated by the fluid friction of the pressurized fluid flowing from the edges of the bearing pockets can result in a noticeable, non-uniform temperature distribution along the length of the hollow roll. The temperature of the hollow roll rises somewhat in the area of the support elements. This may have not only undesirable direct consequences for the treatment of the web, i.e. with respect to temperature, but also indirect consequences, such as the diameter of the hollow roll being increased by thermal expansion in the area of the support elements.
It can be proven that the frictional work occurring at individual support elements having equal gap height h, i.e., equal thickness of the fluid film between the edge of the bearing pockets and the inner circumference of the hollow roll, as the pressurized fluid flows from the bearing pocket, is, in principle, the same. However, there are some second order interference parameters that have a disrupting influence on the prevailing temperature.
At different pressures and constant gap height, different quantities of fluid flow from the pockets. If the frictional work--theoretically equal for all of the support elements--acts upon a small amount of liquid as it flows from the bearing pockets at low pressures, a more intensive heating will occur than if the same frictional work distributes itself over a large amount of liquid, which then is heated to a lesser degree. Furthermore, as the pressurized fluid flows through the narrow gap, hydraulic energy released at the support element is transformed into heat. In addition, the change in viscosity that occurs at the edges of the bearing pockets, due to heating of the pressurized fluid during flow through the gap, also has an effect. Lastly, another factor is speed. At slow speeds, although the oil in the gap may indeed be hotter than at faster speeds, the heat transfer is less. At greater speeds, the oil in the gap does not become as hot, but it transfers more heat to the inner circumference of the hollow roll because of its higher flow velocity.
All of the above effects are particularly significant in disrupting the existing temperature distribution when the line force distribution is changed. That is, the temperature distribution is no longer independent of the line force distribution if a variation in the force exerted by an individual support element necessarily changes the temperature in the area of this support element as well. The totality of all of the effects discussed above is encompassed by the term "fluid friction".